MIPS64: Fix Word32Compare turbofan operator implementation when comparing signed with unsigned operand

MIPS64 doesn't support Word32 compare instructions. Instead it relies
that the values in registers are correctly sign-extended and uses
Word64 comparison instead. This behavior is correct in most cases,
but doesn't work when comparing signed with unsigned operands.
The solution proposed here tries to match a comparison of signed
with unsigned operand, and perform Word32Compare simulation only
in those cases. Unfortunately, the solution is not complete because
it might skip cases where Word32 compare simulation is needed, so
basically it is a hack.

BUG=
TEST=mjsunit/compiler/uint32

Review-Url: https://codereview.chromium.org/2391393003
Cr-Commit-Position: refs/heads/master@{#40398}